Genetic analyses of xenotropic murine leukemia viruses in inbred mouse strains have shown that these viruses may be expressed as infectious virus or as virus-coded antigens (gp7-'s termed XenCSA) on the surface of normal lymphocytes. NZB mice have two independently segregating dominant loci for infectious virus expression and apparently one coding for XenCSA in the absence of infectious virus. The levels of virus expression as infectious virus and XenCSA in NZB lymphocytes is governed by genes affecting lymphocyte differentiation. In genetic crosses of mice exhibiting high (DBA/2) and low (C57BL/6) levels of XenCSA, a single semidominant gene on chromosome 4 at or near Fv-l has the predominant effect on XenCSA levels.